Method of speed control of compressor and control arrangement using the method

ABSTRACT

The invention concerns a method for speed control of a compressor, particularly a refrigeration compressor, and a control arrangement using this method. The speed control is effected in that a control arrangement varies the speed of an electric motor in dependence of simple ON/OFF signals from a thermostat placed in the surroundings to be cooled. The method according to the invention is characterized in that the starting speed of the compressor in a following ON period is reduced in relation to the final speed in the previous ON period. A continuous reduction of the starting speed of each ON period results in a self-regulating control giving long compressor operation times and an averagely low speed resulting in energy savings. A control arrangement using this method is also described.

The invention concerns a method for speed control of a compressor,particularly a refrigeration compressor, and a control arrangement usingthis method. The speed control is effected in that a control arrangementvaries the speed of an electric motor in dependence of simple ON/OFFsignals from a thermostat placed in the surroundings to be cooled.

The task on which the application is based, is the development of acontrol method and a control arrangement for a variable speedcompressor, which is simple compared with the state of the art, andwhich can be fitted into existing constructions, e.g. refrigerators,which normally operated at fixed speed, but which can be changed tooperation with variable speed by a quick intervention of a servicetechnician. It is known to control the speed of compressors independence of the pressure conditions in the refrigeration system or independence of electronic temperature signals, but a common feature ofthese solutions is that they require relatively expensive pressure andtemperature meters, and can only in rare cases directly replace existingsolutions.

U.S. Pat. No. 5,410,230 describes a compressor control with an ON/OFFthermostat controlling the motor speed on the basis of the cyclicparameter, i.e. the duty cycle. The ON and OFF times of the thermostatare measured, and the duty cycle is calculated. If the duty cycle islower than a predetermined value, the starting speed in the next cycleis reduced, and if it is higher, the starting speed is increased. Whenthe thermostat is closed and the compressor operates, the motor speed isramped upwards during the ON period, and correspondingly the motor speedis ramped downwards during the OFF period of the thermostat.

The control described in U.S. Pat. No. 5,410,230 is developed with aview to sawing a control component, e.g. a temperature controllingmicroprocessor, and in stead of having a price increasing component likea microprocessor measure and treat a temperature signal from anelectronic thermostat, U.S. Pat. No. 5,410,230 uses a cheaper solutionwith an ON/OFF thermostat. However, the control method described isrelatively complicated, as both ON and OFF times are measured, and atleast three speed calculations are made during an ON/OFF cycle, viz. afirst calculation of the starting speed, then a second continuouscalculation of the speed in the ON period and a third calculation of thespeed in the OFF period. Besides, the described method involving speedcontrol during both the ON and the OFF period causes a relatively largeenergy consumption.

One of the purposes is thus to develop a control method and a controlarrangement for a compressor having a lower energy consumption than thecontrols known from the state of the art.

Another purpose of the invention is to make a control method and acontrol arrangement for a compressor, which can be directly integratedin existing constructions, by which existing elements, such as an ON/OFFthermostat, are used for compressor speed control.

According to the invention, this is done by letting the conventionalON/OFF thermostat, which are already available in large numbers in therefrigerators, control the compressor speed so that the starting speedin a following ON period is reduced in relation to the final speed in aprevious ON period by making the control arrangement subtract apredetermined speed from the final speed in the previous ON period andletting the result of this calculation be the starting speed in thefollowing ON period, through which the speed is either increased or keptconstant.

Using this control method enables an adaptive control, whichautomatically lets the motor operate at the speed required, whichresults in the refrigeration compressor working with averagely lowspeeds during long operation periods, which gives a low energyconsumption. The operation or ON time is a function of the size of thesubtracted rpm and the time it takes to increase the motor speed.

The invention describes in a first embodiment, how the speed in anactual ON period is measured, and compared by the control arrangementwith the stored final speed from the previous ON period. If the actuallymeasured speed is higher than the stored speed, the result of thecomparison could be that the speed is increased faster than earlier.

To secure that the average motor speed is decreasing, the starting speedof the following ON period is reduced by a fixed value. It is possibleto make the value to be subtracted variable, e.g. to fix it independence of the duration of the ON period, but a particularly simplesolution is obtained by keeping the subtrahend constant from ON periodto ON period. If, in the actual ON period, the thermostat breaks beforethe registered final speed of the previous ON period is reached, thestarting speed in the following ON period will be lower than thestarting speed in the actual ON period. If, however, the thermostatbreaks after that the actual speed has exceeded the registered finalspeed of the previous ON period, the starting speed of the following ONperiod will be higher than the starting speed of the actual ON period.

The specification describes how a suddenly occurring refrigerationdemand can be satisfied by setting the compressor motor speed atmaximum, when a certain limit value is exceeded. This limit can beeither an operation time or a speed change.

According to another embodiment of the invention a first time t1 and asecond time t2 are introduced in addition to the fixed speedsubtraction, at which times the control arrangement changes thecompressor motor speed.

Variable speed control is effected using the method according to theinvention. The variable speed control is characterised in that a controlarrangement subtracts a predetermined speed from the final speed of aprevious ON period and makes the result of this subtraction the startingspeed in a following ON period, through which the speed is eitherincreased or kept constant.

Also included in the invention is a control method, in which the ON timeis measured and compared with the two times, t1 and t2, and the controlarrangement reduces the motor speed, if the thermostat breaks before afirst time t1, maintains the motor speed, if the thermostat breaks afterthe first time t1 but before a second time t2, but increases the motorspeed, if the thermostat has not broken until the time t2.

In the following the invention is described on the basis of thefollowing figures:

FIG. 1 shows a principal sketch of a compressor control arrangement

FIG. 2 shows a speed/time diagram of a first embodiment

FIG. 3 shows a speed/time diagram of a second embodiment

FIG. 4 shows a speed/time diagram of a third embodiment

The control arrangement shown in FIG. 1 is advantageously fitted on thecompressor unit 8 itself. As control and compressor unit are thusintegrated, replacement of a fixed speed compressor by an electronicallycontrolled variable speed compressor can easily be made.

In a room 1 to be cooled, a thermostat 2 is fitted, which breaks ormakes the thermostat contact set 3 in dependence of the roomtemperature. The thermostat 2 is inserted in series with the supplymains. The power part of the control arrangement consists of thecomponents 4, 5 and 6. A rectifier 4 converts the AC voltage of themains to a DC voltage supplied to the intermediary circuit 5 containingan intermediary circuit capacitor 7. The inverter part 6 converts, in aknown way, the DC voltage via pulse width modulation to an AC voltagesupplied to the motor compressor unit 8, which comprises an electricmotor 9 and a compressor 10. The compressor circulates refrigerantthrough a capacitor and an evaporator in a not shown refrigerationcircuit with the purpose of controlling the temperature in room 1. Acontrol arrangement 11 controls and monitors the motor/compressor unitcontrol. The rectifier is controlled via the connection 17, so that theintermediary circuit voltage amplitude can vary. The inverter 6 isoperated via the connection 18, comprising, in a known way, six wiresconnected to the switches in the inverter. On the three motor wires theback-EMF, produced in each phase on rotation of the permanent-magneticrotor of the motor, are measured. The back-EMF signals are sent via thewires 14, 15, 16 to the control arrangement and used for determining thecommutation times.

A temperature sensor 13 is arranged on the inverter cooling plate, andvia connection 20 the signal is led to the control arrangementdisconnecting the motor at a temperature of 100 degrees centigrade. Acurrent measuring resistor 12 is inserted in the minus conductor of theintermediary circuit. Via connection 19 the control arrangement measuresthe current amplitude, and, in case of a too high amplitude, disconnectsthe motor. Via an AC-DC converter 23 and the connections 21 and 22 thecontrol arrangement 11 is supplied with energy from the supply mains. Aswitch 24 has a first position, in which the control is self-regulating(AEO), and a second position enabling external speed control via afrequency signal (n_(ref)).

In the following it is assumed that via the switch 24 the control is inthe Automatic Energy Optimisation (AEO) mode. The control works asfollows, described on the basis of a first embodiment.

A user has set the thermostat 2 at a desired temperature. Onrefrigeration demand the thermostat switch 3 makes. Making and breakingof the thermostat are detected by the control arrangement 11 bymonitoring the mains voltage on conductors 25 and 26. In the first ONperiod the control arrangement lets the motor start at 2600 rpm, whichis in the lower end of the working range 2000 to 3500 rpm. The speed isincreased during the ON period, either continuously or in steps as shownin FIG. 2, until the thermostat breaks. During the period P1 thecompressor has reached a final speed of 2900 rpm. When the thermostatbreaks (OFF), the system is idle until the thermostat makes (ON) again.In the next period P₂ the compressor starts at a speed, which is lowerthan the final speed of the previous period, because a speed of e.g. 400rpm has been subtracted from the final speed. With a starting speed of2500 rpm the compressor is ramped up during the rest of the ON period,increasing the speed by 15 rpm each minute (in FIG. 2 the steps areshown larger to make the case clear). The speed is increased until thethermostat breaks or until the maximum speed of 3500 rpm is reached,which will happen after nearly 67 minutes operation. Then the speed iskept at 3500 rpm. If, during the ON period a limit value is exceeded,e.g. if the speed change during the ON period is higher than 800 rpm,without the thermostat breaking, the compressor motor is set at maximumspeed. Thus a large refrigeration demand suddenly occurring is takeninto consideration. The final speed of period P2 is 2800 rpm, so thestarting speed of the next period P3 is 2400 rpm. FIG. 2 shows asituation with decreasing refrigeration demand, i.e. both final speedand starting speed are lower, when compared with the previous ON period.The inventive control automatically finds a working point with anaveragely low speed and low energy consumption.

The speed ramp is described as a step ramp, but other ramp profiles arepossible. E.g. the actually measured speed during the ON period can becompared with the final speed of the previous period, assuming that thisspeed is stored in a memory. If the actually measured speed exceeds thefinal speed of the previous period, the slope of the ramp in the actualON period can be increased. On the basis of a comparison between thestored speed and the actually measured speed, it can be decided what thestarting speed for the following ON period shall be and how the speedprofile for the actual or the following period shall look.

In a second embodiment, shown in FIG. 3, a fixed speed is alsosubtracted at the beginning of each ON period. FIG. 3 shows the speedreference signal reaching the rectifier via the connection 17 (FIG. 1),controlling the intermediary circuit voltage to the desired level. Inrelation to the first embodiment, threshold values have been introducedin the ON period, in the form of two times t1 and t2, which can be fixedor proportional to the duration of the latest OFF period. The durationof the latest OFF period holds information about the time constant inthe refrigeration system, and the times t1 and t2 can thus be expressedby a number of OFF periods.

A speed is subtracted from the latest generated speed reference, i.e.the final speed of the previous ON period, after which the compressoroperates at a first speed. The control arrangement 11 comprises acounter counting towards a first time t1. The times are stored in amemory in the control arrangement. If the thermostat has not broken atthe time t1, the speed is increased by a adding a predetermined value.If the thermostat has still not broken at the time t2, the speed isincreased further, and this can happen stepwise through the remaining ONperiod or with one single speed step, after which a constant speed ismaintained for the rest of the period. In the following period P2 thestarting speed is again reduced by an amount, and it can be seen thatthe thermostat breaks before the time t1, which means decreasingrefrigeration requirement. In the period P3 the starting speed inaccordance with the invention is decreased in relation to the finalspeed in P2.

Reducing the starting speed for each ON period causes the compressor torun in long operation periods with a speed, which is on an averagesubstantially lower than that of conventionally ON/OFF operatedcompressors, and this results in energy savings.

FIG. 4 illustrates a control strategy similar to that of the secondembodiment, also using two threshold values t1 and t2. The controlarrangement reduces the motor starting speed in the following ON period,if the thermostat breaks before the first time t1, but increases thecompressor speed, if the thermostat breaks after the time t2. If,however, the thermostat breaks between the times t1 and t2, the speed isunchanged. In the period P1 the compressor has just started, andtherefore has an operation time exceeding the upper time limit t2. Thismeans that the speed is increased from the start of the next period P2.As shown in the figure, the speed can be increased by one contributionfrom the start of the cycle, after which it is kept constant during therest of the ON period, or the speed can be changed stepwise during thewhole ON period. In period P2 the thermostat does not break until afterthe time t1 but before the time 2, therefore the starting speed in thefollowing ON period is unchanged. In the period P3 the thermostat breaksbefore the time t1, and thus indicates reduced refrigeration requirementcausing the starting speed in P4 to be reduced by an amount, which couldbe 400 rpm. As shown, the speed change does not take place until thefollowing ON period, but of course the change can also be made in theactual ON period, as shown in embodiment 2.

What is claimed is:
 1. Method for controlling motor speed in acompressor used in cooling an area such as a room, the compressor havingan electronic control arrangement, which, from a thermostat having ONand OFF states, receives a signal depending on the temperature in theroom to be cooled, and the compressor having a variable starting speed,the method comprising the steps of reducing the starting speed in asubsequent ON period in relation to the final speed in a previous ONperiod by making the control arrangement subtract a predetermined speedfrom the final speed in the previous ON period and using the result ofthis calculation as the starting speed in the subsequent ON period, andduring the ON period either increasing or keeping the speed constant. 2.Method according to claim 1, in which the control arrangement stores thefinal speed from the previous ON period in a memory and measures thecompressor speed during an actual ON period, compares of these twospeeds, and determines the starting in the actual or the following ONperiod.
 3. Method according to claim 1, in which the predetermined speedsubtracted has a constant value such so that the starting speed in thesubsequent ON period is lower than the starting speed in the previous ONperiod if the thermostat delivers an OFF state signal in the previous ONperiod before the actual compressor speed is equal to the starting speedin the previous ON period plus the constant value.
 4. Method accordingto claim 1, including the step of setting the speed during an ON periodat a maximum if speed change during this ON period exceeds apredetermined limit value.
 5. Method according to claim 1, including thestep of setting the speed during an ON period at maximum if totaloperation time during the ON period exceeds a fixed limit value. 6.Method according to claim 1, including the steps of increasing the speedat a first time t1 and then again at a second time t1.
 7. Methodaccording to claim 6, in which the speed is kept constant at a firstspeed until the time t1, whereafter the speed is raised and keptconstant at a second speed until the time t2, after which the speed isincreased at a constant rate.
 8. Variable speed control for regulatingthe motor speed in a refrigeration compressor used in cooling an areasuch as a room, comprising an electronic control arrangement forreceiving a temperature signal from a thermostat placed in the room tobe cooled, the thermostat having ON and OFF states, the controlarrangement having means to reduce the starting speed of the compressorin a subsequent ON period in relation to the final speed in the previousON period by subtracting a predetermined speed from the final speed inthe previous ON period, and using the result of this calculation as thestarting speed in the following ON period.
 9. Method for speed controlof a motor in a compressor by means of an electronic control arrangementreceiving a temperature signal from a thermostat having ON and OFFstates, and in which the speed determined by the control arrangement isa function of the duration of at least one of the ON period and OFFperiod, comprising steps of reducing the motor speed if the thermostatsends an OFF state signal before a first time t1, maintaining the motorspeed if the thermostat sends an OFF state signal after the first timet1 but before a second time t2, and increasing the motor speed if thethermostat has not sent an OFF state signal by the time t2.
 10. Methodaccording to claim 9, including the further steps of keeping the motorspeed constant in the ON period and changing the motor speed in asubsequent ON period.